Summary

R/V Maurice Ewing cruise
EW-9607 formed the major data acquisition phase of SIGMA (Seismic Investigation
of the Greenland MArgin), a joint U.S.-Danish investigation of the deep crustal
structure of the Southeast Greenland continental margin. Previous seismic data
indicate that Southeast Greenland is a volcanic rifted margin that contains
significant thicknesses of mafic igneous material emplaced during early Tertiary
continental breakup. The purpose of the SIGMA survey was to provide critical
missing information on (1) the thickness, velocity structure, and composition
of the Tertiary igneous crust; (2) variations in these properties both across
the margin and along the margin at increasing distance from the Iceland plume
track; and (3) the detailed structure of the continent-ocean boundary. This
was achieved by acquiring seismic data along four margin-crossing transacts,
one along the Greenland-Iceland Ridge and three at successively increasing distances
of 200-1100 km from the presumed plume track. The Ewing fired over 33,000 shots
from its 20-gun tuned airgun source into an array of offshore and onshore seismic
receivers: the Ewing`s 4-km-long multichannel streamer, eleven Woods Hole Oceanographic
Institution ocean-bottom hydrophones (OBH), eight U.S. Geological Survey ocean-bottom
seismometers (OBS), and 35 Reftek portable seismometers deployed on two transacts
in Greenland and one in Iceland. Despite several gales, two storms, heavy fog,
cold water, and numerous icebergs, the operational goals of the experiment were
largely attained. Sixty-nine deployments and 68 recoveries of OBH/S were accomplished
(one OBH was lost during recovery); over 1300 km of MCS data were acquired in
four deployments of the multichannel streamer; and four sonobuoys were launched.
Preliminary velocity models were produced shipboard for two of the offshore
lines. Migrated brute stacks of all MCS lines were produced in near-real-time
shipboard, and further shipboard MCS processing, including velocity picldng
and multiple suppression, provided improved crustal images. The data collected
during this experiment will provide key information on the degree of structural
and magmatic symmetry on conjugate VRM segments and on the influence of the
Iceland hotspot on margm magmatism during and immediately after continental
breakup. This information will provide an important framework for interpreting
geological and drilling results on the margin and for constraining then-nomechanical
models of VRM evolution. This study is a joint venture between US and Danish
co-investigators, including data acquisition, processing, interpretation, and
project financing.

Scientific Objectives

The SIGMA (Seismic Investigation of the Greenland Margin) project is designed
to make accurate measurements of crustal thickness, velocity structure, and
seismic reflectivity along the hotspot-influenced volcanic rifted margin (VRM)
off Southeast Greenland.

VRM`s are characterized
by a prism of igneous rocks, several times thicker than normal oceanic crust,
that occupies the continent-ocean transition zone in an 80- to 150-km-wide
belt and extends in some regions more than 1500 km along strike. This thick
igneous crust has two characteristics in seismic data: a seaward-dipping reflector
sequence interpreted as subaerially erupted basalt flows and intercalated
volcaniclastics, and a high-velocity lower crust with P-wave velocities (7.2-7.6
km/s) suggestive of mafic/ultrainafic intrusive rocks. Several models for
the thermal and mechanical processes involved in the formation of VRM have
been proposed, including decompression melting during passive upwelling near
a mantle plume; actively upwelling plume heads impinging on the base of the
lithosphere; enhanced upper mantle convection driven by steep, cold lithospheric
edges adjacent to the rift; and hot upper mantle due to non-plume "hot cells"
or insulation by supercontinents.

SIGMA consists of four
transacts that systematically sample the structure of the SE Greenland margin
and the continent-ocean transition at increasing distance from the Iceland
hotspot track. The resulting data will provide answers to several questions
regarding the SE Greenland VRM, including:

What is the structure of the transition from continental to thick igneous crust, and thence to
normal oceanic crust? Is the transition abrupt or gradual? To what extent
does faulting play a role? Does the abruptness of the continent-ocean transition
change with distance from the Iceland plume?

What was the total
volume of magmatism during continental breakup on the SE Greenland margin
and its conjugates, and how does it vary in space and time? How does this
magmatism relate to distance from the Iceland plume and to its temporal
magmatic budget? What is the proportion of plutonic to volcanic rocks, and
how does this vary with distance from the hotspot track and with total crustal
thickness?

Does high-velocity
lower crust exist beneath the margin, and if so, is there any evidence that
its composition, thickness, and distribution change along strike? How might
such changes relate to variations in melting conditions (temperature and
degree of melting) with distance from the plume?

Is the structure of
the SE Greenland margin symmetric with its conjugate margins on the Hatton/Rockall
Bank and Iceland-Faeroes Ridge? What combinations of pure shear and simple
shear processes might explain the conjugate structures?

Operational Objectives

The SIGMA cruise comprised
a seismic survey of the Southeast Greenland rifted margin along four transects,
one along the Greenland-Iceland Ridge and three at successively greater distances
from the presumed track of the Iceland hotspot. Wide-angle seismic data were
recorded by 37 onshore Reftek portable seismogaphs deployed along three profiles,
by 11 Woods Hole Oceanographic Institution OBH and 8 U.S. Geological Survey
OBS offshore, and vertical-incidence data were recorded by the Ewing`s 4-km-long
towed hydrophone streamer. Shots were fired using the Ewing`s 130-liter (8495
cu. in.) airgun array and by chemical explosions detonated by the onshore
team on Transect 11.

The primary operational
goals were to:

Deploy and recover 19 OBH/S on four profiles across the Southeast Greenland rifted margin.

Fire the 20-gun array into the OBH/S and MCS streamer on all four transacts, and into onshore
arrays on Transects II and III in Greenland and Transect I in Iceland.

Record on onshore Refteks and, if possible, OBH and OBS, chemical explosions detonated by the onshore
team in Kangerdlugssuagsiak Fjord and off the coast on Transect III.

Record sonobuoy data on the eastern part of the Greenland-Iceland Ridge profile.

Maintain good communications with the DLC/WHOI onshore team in Greenland, with the Cambridge onshore
team in Iceland, and with onshore PI`s at critical decision points.

Produce SEGY archive files for the OBS/H data and sonobuoy data.

Produce pseudo-real-time stacks of all MCS data and plots of shot gathers to monitor data quality.